Since the xDSL (Digital Subscriber Line) technology came to the world, it has been deployed worldwide because it can provide broadband services through conventional resources of copper lines so as to effectively protect the investments of operators and to provide broadband services for the subscribers. In the xDSL family, the personal user-oriented ASDL (Asymmetric Digital Subscriber Line), which targets the residential service, has an absolute dominance in broadband access applications for personal users because it is very suitable for the asymmetric characteristic of traffics for personal broadband access services, that is, the amount, of data from a Central Office (CO) to a Customer Premise Equipment (CPE) (called the downlink direction) is much greater than that of the uplink direction (from the CPE to the CO). At present, multimedia services provided by ADSL, such as a high-speed network access, a video service and the like, has been utilised widely, and the ADSL services and the Plain Old Telephone Services (POTS) have together become important services provided by the Public Switched Telephone Network (PSTN). Furthermore, with an increasing demand of the users for access bandwidths, the access technology of VDSL (Very-high Digital Subscriber Line) will be applied which is capable of providing a higher bandwidth and a more flexible distribution of uplink and downlink bandwidths.
The ADSL and the VDSL utilize a higher frequency band above the conventional band for telephone signals (below 3.4 KHz). Here, the band for the ADSL ranges from 25.875 KHz to 1.104 KHz, the band for the ADSL2+ ranges from 25.875 KHz to 2208 KHz, and the VDSL may use the band of 25 KHz to 30 MHz. An xDSL signal and a POTS signal may be mixed and transmit over the same telephone line simultaneously since they utilise different frequency bands, and a simple filter can be adopted to separate xDSL and POTs signal from mixed signals. For the ADSL and the VDSL, such a filter is also called a splitter, and the output of the splitter is a POTS service signal is split from, that is, an output of the splitter is connected with the PSTN. An equipment providing in a centralized manner a plurality of ADSL and VDSL services within a communication network is called a DSL Access Multiplexer (DSLAM). A reference model for the ADSL and the VDSL system is shown in FIG. 1. Here, the output of the DSLAM may be connected to a conventional telephone and an xTU-T (xDSL Transmission Unit-Remote) respectively, an xTU-C (xDSL Transmission Unit-Central) is provided in the DSLAM, and the DSLAM is also connected with, a Network Management System (NMS).
As the application scale of the xDSL expands sharply, the system maintenance becomes increasingly important. The requirement of the xDSL for transmission lines is much higher than that, of the POTS, since the xDSL service utilizes a higher hand than the POTS service. In addition, the twisted pair line was originally designed for the POTS service, so not all the lines capable of providing a telephone service may be suitable for the xDSL. Furthermore, the corresponding PSTN equipments become more complicated due to the terminals of the DSLAM and the xDSL. Therefore, the provisions of a fault analysis and positioning system as well as means far the system maintenance are very important.
An automatic test system for an xDSL system mainly includes a line-access device and a test circuit. The automatic test system for an xDSL system generally provides the test functions including a test for a central office unit (i.e., an xTU-C), a test for an xDSL remote unit (i.e. an xTU-R) and a test for a line. The central office unit, the line and the remote unit have to be switched onto the test system (i.e., the test circuit) respectively in order to carry out the above respective test functions. This process of switching is called line-access. Taking the central office unit as a reference, the test for the central office unit is called an inner-test, and the test for the line and the remote unit is called an outer-test. Another test is to monitor an xDSL line which works normally without any influence on normal operations of the xDSL service and the POTS service.
In general, there are two sets of buses for use in an automatic test system for a DSLAM, one of which is called an inner-test bus, and the other of which, is called an outer-test bus. The so-called line-access is actually a process for connecting a port to be test onto the outer-test or inner-test bus. Since a POTS service is also carried on an xDSL line, the line-access in the test process should influence the narrowband POTS service as least as possible. Furthermore, in view of a line renting relationship between the Incumbent Local Exchange Carriers (ILECs) and the Competitive Local Exchange Carriers (CLECs) in some countries (a CLEC may rent a line of an ILEC for providing a broadband service, and the ILEC also provides a POTS service on the same line), and some ILECs may have their own narrowband test apparatus, in this case, when the ILEC tests a line, the broadband portion should be able to be bypassed without any influence thereupon.
In addition, in order to improve the service transmission quality for a communication system, it is necessary to provide a backup mechanism for a port in the communication system, so a switching can be performed onto a backup port when the corresponding port-fails, so as to ensure that a communication service will not be interrupted.
Function modules for implementing the above test line-access and backup functions are generally implemented by a relay matrix connected in the transmission line. The use of numerous relays may increase the implementation cost and also the area of a corresponding Printed Circuit Board (PCB), which results in a reduced port density. Therefore, it is impossible to meet the requirements for the broadband communication and the test.